Myelin is a specialized membrane that ensheathes neuronal axons, promoting efficient nerve impulse transmission (Morell and Quarles (1999) Basic Neurochemistry: molecular, cellular, and medical aspects. In Siegel G J, ed. Myelin Formation, Structure, and Biochemistry. Lippincott-Raven Publishers, 79-83). Due to its important biological functions in the normal central nervous system (CNS) and its vulnerability in disease, several techniques have been developed to visualize and characterize myelin histopathology. These can be broadly divided into those based upon antibody immunohistochemistry (IHC) (Horton and Hocking (1997) Cereb. Cortex 7:166-177) and more traditional histochemical procedures. The classic histochemical stains include luxol fast blue MBN (Kluver and Barrera (1953) J Neurosci Methods 153: 135-146; Presnell and Schreibman (1997) Humanson's Animal Tissue Techniques, 5th ed.; Kiernan (1999) Histological and Histochemical Methods: Theory and practice, 3rd ed.; Bancroft and Gamble (2002), Theory and Practice of Histological Techniques, 5 ed. and Sudan Black B (Lison and Dagnelie (1935) Bull. d'Histologie Appliquee 12: 85-91). Traditional chromogenic methods also include the Palweigert method ((Weigert (1884) Fortschr Deutsch Med 2: 190-192, (1885) Fortschr Deutsch Med 3:236-239; Clark and Ward (1934) Stain Technol 54:13-16), the Weil stain (Weil (1928) Arch Neurol Psychiatry 20:392-393; Berube et al. (1965) Stain Technol 40:53-62)), the Loyez method (Cook (1974) Manual of Histological Demonstration Methods, 5th ed.), and a method based on horse serum followed by subsequent reaction with diaminobenzidine (McNally and Peters (1998) J Histochem Cytochem 46:541-545). In addition, modified silver stains including the Gallyas method (Pistorio et al. (2005) J Neurosci Methods 153: 135-146) and Schmued's gold chloride technique (Schmued and Slikker (1999) Brain Res 837:289-297) have also been used as simple, high-resolution histochemical markers of myelin. More recently, fluoromyelin (Kanaan et al. (2005) Am J Physiol Regul Integr Comp Physiol 290:R1105-1114) and NIM (Xiang et al. (2005) J Histochem Cytochem 53:1511-1516) were introduced as novel myelin dyes, which enable quick and selective labeling of myelin in brain tissue sections. Although these myelin-staining techniques are widely used in vitro, none can be applied in vivo due to impermeability of the blood-brain barrier (BBB). The lack of in vivo radioligands has limited the progress of myelin imaging and hindered efficacy evaluation of novel myelin repair therapies during their development.